Channel reciprocity character, namely the symmetry character between uplink (UL) and downlink (DL) frequencies, has good application prospect in LTE-A (Advanced—Long Term Evolution) TDD (Time Division Duplex) system. The industry commonly accepts the assumption of reciprocity between uplink and downlink, and uses the assumption to estimate the channel effectively. However, in practice, for the reason of the difference of RF (Radio Frequency) circuits between the receiver and the transmitter, especially the difference of RF circuits between the receiver and the transmitter at base station side, the reciprocity between uplink and downlink in TDD system is very hard to be guaranteed. For the TDD based system with reciprocity between uplink and downlink, the system performance is highly sensitive to uplink/downlink channel reciprocity errors, slight reciprocity errors between uplink and downlink might cause significant performance degradation. Therefore, reciprocity calibration in TDD system attracts the interest of the industry.
The calibration for OTA (Over The Air) interface of TDD system with reciprocity between uplink and downlink becomes an effective way to guarantee reciprocity between uplink and downlink of TDD system, since there is no need to introduce additional hardware devices. However, the accuracy of OTA calibration mainly depends on channel estimations between uplink and downlink, and thus how to perform user terminal (also called user equipment or UE) selection is an important problem, especially for CoMP (Coordinated Multi Point) system, which user terminals are selected for calibration is a problem urgently to be solved.
Exiting time division duplex technique proposes that the selected user terminals should locate near the center of coordinated multi-point cell cluster, which means only user terminal locations are taken into consideration for user terminal selections.